In recent years, a great amount of image information is being circulated with the progress of multimedia technology. Liquid crystal displays have sprung into wide use as means for displaying such image information. This is because high contrast and wide viewing angle liquid crystal displays have been developed and put to practical use by virtue of the progress of liquid crystal technology. At present, the display capability of liquid crystal displays has reached a level comparable to that of CRT displays.
Liquid crystal displays, however, involve a problem that they are not suited to motion picture display because the responsiveness thereof is insufficient for motion picture display. Specifically, though the NTSC (National Television System Committee) system now in force requires that liquid crystal make a response within one frame period (16.7 msec), existing liquid crystal displays take 100 msec or longer to make a response at a portion between adjacent levels of gray in a multi-level gray scale display. For this reason, there occurs a phenomenon that an image blurs in a motion picture display. Particularly at a portion between adjacent levels of gray in a region at which the driving voltage is low, a response is considerably delayed and, hence, a favorable motion picture display cannot be realized.
In this respect, many attempts have been made to make higher the responsiveness of liquid crystal displays. Though various liquid crystal display systems for high-speed response have been summarized by Wu et al. (C. S. Wu and S. T. Wu, SPIE, 1665, 250 (1992)), the number of such systems expected to have response characteristics required for motion picture display is limited at present.
Presently, liquid crystal displays comprising an OCB embodiment liquid crystal display element, a ferroelectric liquid crystal display element or an antiferroelectric liquid crystal display element are considered to be promising as liquid crystal displays having such high-speed response as to be suited for motion picture display.
Among such liquid crystal display elements, ferroelectric liquid crystal display elements and antiferroelectric liquid crystal display elements, which are of a layered structure, involve many problems in terms of practical use such as low impact resistance, narrow service temperature range, and high temperature dependence of characteristics. For this reason, attention is actually focused on OCB embodiment liquid crystal display elements using nematic liquid crystal.
In 1983, J. P. Bos demonstrated the high-speed response of such an OCB embodiment liquid crystal display element. Thereafter, OCB embodiment liquid crystal display elements were proved to exhibit both wide viewing angle and high-speed response compatibly with each other if it is provided with a retardation plate. Since then, research and development of such OCB embodiment liquid crystal display elements has become active.
FIG. 24 is a sectional view schematically showing the construction of a conventional OCB embodiment liquid crystal display element. As shown in FIG. 24, the OCB embodiment liquid crystal display element includes a glass substrate 1 formed with a transparent electrode 2 on the underside thereof, a glass substrate 8 formed with a transparent electrode 7 on the upper side thereof, and a liquid crystal layer 4 interposed between these glass substrates 1 and 8. An alignment film 3 is formed on the underside of the transparent electrode 2, while an alignment film 6 formed on the upper side of the transparent electrode 7. Liquid crystal molecules have been filled into a gap between these alignment films 3 and 6 to be formed into a liquid crystal layer 4. The alignment layers 3 and 6 have been subjected to alignment treatment to align the liquid crystal molecules in parallel with one another and in the same direction. The thickness of the liquid crystal layer 4 is maintained with spacers 5.
The glass substrate 1 is provided with a sheet polarizer 13 on the upper side thereof, while the glass substrate 8 provided with a sheet polarizer 16 on the underside thereof, the sheet polarizers 13 and 16 being arranged in a cross nicol position. Further, a retardation plate 17 is provided between the sheet polarizer 13 and the glass substrate 1, while a retardation plate 18 provided between the sheet polarizer 16 and the glass substrate 8. A negative retardation plate having a hybrid-aligned primary axis is employed for each of the retardation plates 17 and 18.
The OCB embodiment liquid crystal display element thus constructed is characterized in that transition of the alignment condition of liquid crystal molecules from a splay alignment 4a to a bend alignment 4b is caused by application of a voltage to allow an image to be displayed with the molecules in the bend alignment condition. Such an OCB embodiment liquid crystal device exhibits considerably improved liquid crystal responsiveness as compared with TN (Twisted Nematic) embodiment liquid crystal display elements and the like and hence can realize a liquid crystal display suited for motion picture display. Further, the provision of the retardation plates 17 and 18 makes it possible to realize low-voltage drive and a wide viewing angle.
Meanwhile, the aforementioned OCB embodiment liquid crystal display element may be constructed to include color filters for the three primary colors (red, green and blue) for realizing a color display. Pixels corresponding to respective color filters for red, green and blue are herein referred to as red pixel(s), green pixel(s) and blue pixel(s), respectively. FIG. 25 is a graph showing wavelength dispersion characteristics in accordance with retardations in a normal direction of liquid crystal layers, respectively, associated with such red pixel, green pixel and blue pixel (hereinafter referred to as normal-direction retardation(s)). FIG. 25 also shows the normal-direction retardation of a negative retardation plate having a hybrid-aligned primary axis, together with the normal-direction retardations of these liquid crystal layers.
In FIG. 25, reference numerals 81, 82 and 83, respectively, indicate wavelength dispersion characteristics in accordance with normal-direction retardations of the liquid crystal layers, respectively, associated with the red pixel, green pixel and blue pixel. Reference numeral 84 indicates the wavelength dispersion characteristic in accordance with normal-direction retardations of the aforementioned negative retardation plate.
As shown in FIG. 25, the normal-direction retardation of the liquid crystal layer associated with the red pixel generally conforms to that of the negative retardation plate in a wavelength region corresponding to red (in the vicinity of 650 nm). Likewise, the normal-direction retardation of the liquid crystal layer associated with the green pixel generally conforms to that of the negative retardation plate in a wavelength region corresponding to blue (in the vicinity of 550 nm). However, the normal-direction retardation of the liquid crystal layer associated with the blue pixel does not conform to that of the negative retardation plate in a wavelength region corresponding to blue (in the vicinity of 450 nm). Thus, there arises a problem that when the OCB embodiment liquid crystal display element makes a black display, the display becomes bluish.